Pros
Flexible hours made juggling life as a full-time student and a part-time employee very easy. Each store environment is greatly influenced by it's GM. I had an amazing GM who worked with my schedule and was a great leader/manager. Then I was transferred to a store where the GM was a jerk with an ego and a military-style work ethic. All of the sudden, my job became hell. Overall, Fastenal is fun, laid back, and was a great part-time job - if you are lucky enough to be placed at a store with a good GM.
Cons
Upper management (the district managers - I worked under two different one during my three-year employment) are the cliche "how did they get that high in the company with how emotionally unintelligent and lack of knowledge?" types. But you do not have to deal with them too often, especially as a part-timer. Work environment is highly dependent on each individual store GM.